258 NATURAL HISTORY OF ORGANIZED BODIES. 



siolog-ical experiment, ■wliile excellent advice is given regarding the disposition 

 of mind wliicli it is necessary to bring to the study of biology. 



Bat, of itself alone, vivisection is insufficient for this pursuit; it can do no more, 

 so to say, than lay bare the phenomenon simultaneously with the organ which 

 is the seat of it ; it reveals U) our senses only what they are capable of perceiv- 

 ing. Now, we have seen that in physics our senses teach us very little, and that 

 it is necessary, at every step, to have recourse to apparatus for analyzing the 

 more delicate phenomena. The same is the case in biology. The electrical 

 phenomena which take place in animals are, in certain cases, directly percepti- 

 ble. The commotion produced by the torpedo and gymnotus have been known 

 from antiquity, but the most sensitive galvanometers have been needed to detect 

 those electric modifications, so weak and yet so important, which accompany the 

 nervous and muscular actions. Du Bois-Reymond and his successors have 

 made known to us an entire new phase of physiology, and one of the most 

 interesting kind. Optical apparatus is indispensable for the exploration of the 

 interior of the eye, as well as for the delicate measurement of the curvatures of 

 each of the refractive mediums which compose it. Thus, while dissection 

 teaches us certain details of the organization, it would nevertheless deceive us 

 by destroying the normal disposition of the parts, had we not the means of 

 studying the living apparatus in situ. 



Anatomy shows us the organs with a definite form and volume ; physiology, 

 on the contrary, teaches us that most of the organs present, in the actions of life, 

 changes both of form and volume, a few of wliich only can be easily perceived. 

 We must resort to instrumental aid for the demonstration of changes too deli- 

 cate for naked vision, Now, micrometry, as is well known, has attained an 

 extraordinary precision in the determination of the diameters of objects extremely 

 minute ; it constitutes one of the principal resources at the disposal of histology, 

 and enables it, in effect, to assign to each element its normal diameter, which is 

 one of its important characteristics. 



As there exists, then, a micrometry by which we can measure the slightest 

 changes in the volume of the organs in living animals, I deem it the more 

 important to indicate the apparatus destined for this purpose, since it is still but 

 little employed, though possessing, in certain cases, very great utility. It Avill be 

 remembered that discussions were lierctofore maintained respecting the dilatation 

 of the arteries under the afflux of the blood propelled into them by the contrac- 

 tion of the ventricle. Some writers contended that the arterial system makes 

 room for the sanguineous Avave by means of an elongation sustained hy the ves- 

 sels, while otliers thought that the arteries, in this act, dilate and lengthen at 

 the same time. 



To resolve this question, ]\[. Flourens conceived the idea of encircling the 

 artery of a living animal with an interrupted ring formed of an elastic spring, which 

 would yield to the dilatation of tlie artery and manifest it by the separation of the 

 two ends of the ring. This separation takes place, in effect, whenever anew dis- 

 charge of blood is received from the heart. But the method is not wholly free from 

 objection. If we suppose the pressure of the elastic ring to produce a slight 

 constriction of the vessel, the latter may simply recover its normal dimension, 

 and in this way, without undergoing dilatation, would separate the ends of the 

 ring which compressed it. ]\I. Poiseuille employed a more rigorous method, 

 which consists in placing the artery which Ave propose to examine in a small 

 box with rigid walls, pierced on one side and the other by a suitable hole. In 

 this box the artery is maintained at such a degree of tension as to preclude any 

 liability to elongation thruugli pressure of the blood. The box is filled with 

 liquid, and is famished at some point in its walls Avith a capillary tube in Avhich 

 the liquid ascends to a determinate level. If the blood-vessel thus enclosed 

 undergoes the slightest augmentation of diameter, it necessarily displaces the 

 liquid of the box, and the level in the capillary tube is seen to rise or descend, 



